Paper products, such as paper towels, napkins and toilet tissue are widely used on a daily basis for a variety of household needs. Typically, such products are formed of a fibrous elongated web which is either packaged in rolls or in a folded stack. The fibrous webs are usually embossed to increase the bulk of the tissue and to improve the absorbency, softness and appearance of the product both as individual sheets, and in providing a uniform stack or roll package. Embossing can also aid in holding superposed plies of a web together. Generally, the embossing apparatus will include one or more rolls having male protuberances thereon for forming the embossed pattern, and a corresponding back-up roll which holds the web against the male embossing roll such that the embossed pattern is imparted to the web as it passes between the nip of the male roll and the backup roll.
In early embossing operations, a fiber roll was utilized as the backup roll, with the fiber roll formed of a hard cloth material. The male roll was formed of metal and included the protuberances engraved thereon. Prior to use of the rolls for embossing, the male roll and backup roll were run together (without a web passing therebetween), with soap and water utilized for lubricating and softening purposes. The male roll and backup roll would be run together until the fiber backup roll took on the female pattern corresponding to the protuberances of the male roll. The use of the rolls in embossing of paper products did not begin until after the female pattern or indentations corresponding to the male roll were achieved. Generally, this would require 24-36 hours of operation. Thus, the fiber roll approach required a great deal of initial start-up time and cost associated with operating the rolls without embossing web products.
In a steel to steel mated embossing approach, male protuberances are provided on a steel male roll, and corresponding female indentations are engraved in a female backup roll. As the web is passed through the nip formed between the two rolls, the male protuberances emboss the web, and are accommodated by the grooves or indentations in the female backup roll. To prevent damage as a result of interference between the protuberances and indentations, a clearance of 0.003-0.007 inches must be provided. Due to the required clearance, the steel to steel approach is not as successful (as other approaches, e.g. rubber to steel as discussed hereinafter) in softening the fibrous product, since the clearance reduces the breaking of fibers or fiber bonds as compared to other approaches in which the web is softened by "working" the web, i.e. by fracturing fibers or fiber bonds in the web.
In rubber to steel embossing, the steel roll is provided with the male protuberances and the web is squeezed against the male roll by a rubber backup roll, as the web passes through the nip. The rubber accommodates the protuberances by virtue of its resilience, and the rubber flows about the protuberances as force is applied to urge the rolls together. However, to ensure that the rubber flows about the protuberances to achieve an acceptable embossed pattern, an extremely large amount of force is required. As production demands have increased, the desirable lengths of such rolls has increased to 80-130 inches in length and sometimes even higher. An extremely large amount of force is required to urge such lengthy rolls together, while ensuring the rubber flows about the protuberances. However, where large amounts of force are applied, the roll may deflect or bend, such that acceptable rubber flow is achieved at the ends, but not in the center portions of the roll.
To prevent or reduce the deflection, very large diameter rolls, for example on the order of 20 inches, are necessary. This can make the rolls extremely costly. In addition, the large amount of force or pressure between the rolls develops a great deal of heat on the rolling contact surfaces. As a result of the heat, the rubber can actually burn off, and over an extended period of time, hardening, cracking and other heat associated wear will occur. As the rubber roll wears, pieces of rubber can actually become dislodged and thrown from the roll, exposing employees to a quite dangerous condition.
Thus, utilizing the conventional rubber to steel arrangement, it is extremely difficult to achieve a uniform embossed pattern along the length of the roll (or across the width of the web) as a result of difficulties in applying sufficient force to cause the rubber to deform about the protuberances along the entire length of the roll, and also as a result of the associated wear on the rubber roll. In addition, the rubber roll can become unsafe and require replacement or maintenance, making the process expensive, particularly since large diameter rubber rolls are required. However, the use of rubber rolls can be desirable in that as the web passes through the rubber to steel nip, the web is enhanced and softened as the sheet is worked, by virtue of the rubber flowing about the male protuberances resulting in breaking of the fibers (or bonds among fibers) extending through the web. Thus, a softer product is produced, as compared to a web which is run through the mated steel to steel arrangement.
In accordance with the present invention, it is desired to provide an embossing method and apparatus which reaps the advantages of rubber to steel embossing, while overcoming the disadvantages of conventional rubber to steel embossing approaches.